Using magnetic force/field for drill bits and other cutting tools

ABSTRACT

A cutting element assembly includes a rotatable cutting element having an axis of rotation extending axially therethrough, a cutting face and a body extending axially downward from the cutting face, at least one retention mechanism disposed adjacent to the rotatable cutting element, and a first magnet disposed at a back face of the rotatable cutting element.

BACKGROUND

Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation. Drill bits in the first category are generally known as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.

Drill bits of the second category are typically referred to as “fixed cutter” or “drag” bits. Drag bits, include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits.

PDC cutters have been used in industrial applications including rock drilling and metal machining for many years. In PDC bits, PDC cutters are received within cutter pockets, which are formed within blades extending from a bit body, and are typically bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.

In a typical PDC cutter, a compact of polycrystalline diamond (“PCD”) (or other superhard material, such as polycrystalline cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamond grains or crystals that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.

An example of a prior art PDC bit having a plurality of cutters with ultra hard working surfaces is shown in FIGS. 1 and 2. The drill bit 100 includes a bit body 110 having a threaded upper pin end 111 and a cutting end 115. The cutting end 115 typically includes a plurality of ribs or blades 120 arranged about the rotational axis L (also referred to as the longitudinal or central axis) of the drill bit and extending radially outward from the bit body 110. Cutting elements, or cutters, 150 are embedded in the blades 120 at predetermined angular orientations and radial locations relative to a working surface and with a desired back rake angle and side rake angle against a formation to be drilled.

A plurality of orifices 116 are positioned on the bit body 110 in the areas between the blades 120, which may be referred to as “gaps” or “fluid courses.” The orifices 116 are commonly adapted to accept nozzles. The orifices 116 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades 120 for lubricating and cooling the drill bit 100, the blades 120 and the cutters 150. The drilling fluid also cleans and removes the cuttings as the drill bit 100 rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters 150 may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 100 toward the surface of a wellbore (not shown).

Referring to FIG. 2, a top view of a prior art PDC bit is shown. The cutting face 118 of the bit shown includes a plurality of blades 120, wherein each blade has a leading side 122 facing the direction of bit rotation, a trailing side 124 (opposite from the leading side), and a top side 126. Each blade includes a plurality of cutting elements or cutters generally disposed radially from the center of cutting face 118 to generally form rows. Certain cutters, although at differing axial positions, may occupy radial positions that are in similar radial position to other cutters on other blades.

A factor in determining the longevity of PDC cutters is the exposure of the cutter to heat. Exposure to heat can cause thermal damage to the diamond table and eventually result in the formation of cracks (due to differences in thermal expansion coefficients) which can lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and conversion of the diamond back into graphite causing rapid abrasive wear. The thermal operating range of conventional PDC cutters is typically 700-750° C. or less.

As mentioned, conventional polycrystalline diamond is stable at temperatures of up to 700-750° C. in air, above which observed increases in temperature may result in permanent damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond is due to the difference in the coefficient of thermal expansion of the binder material, cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. Damage may also be due to graphite formation at diamond-diamond necks leading to loss of microstructural integrity and strength loss, at extremely high temperatures.

In conventional drag bits, PDC cutters are fixed onto the surface of the bit such that a common cutting surface contacts the formation during drilling. Over time and/or when drilling certain hard but not necessarily highly abrasive rock formations, the edge of the working surface on a cutting element that constantly contacts the formation begins to wear down, forming a local wear flat, or an area worn disproportionately to the remainder of the cutting element. Local wear flats may result in longer drilling times due to a reduced ability of the drill bit to effectively penetrate the work material and a loss of rate of penetration caused by dulling of edge of the cutting element. That is, the worn PDC cutter acts as a friction bearing surface that generates heat, which accelerates the wear of the PDC cutter and slows the penetration rate of the drill. Such flat surfaces effectively stop or severely reduce the rate of formation cutting because the conventional PDC cutters are not able to adequately engage and efficiently remove the formation material from the area of contact. Additionally, the cutters are typically under constant thermal and mechanical load. As a result, heat builds up along the cutting surface, and results in cutting element fracture. When a cutting element breaks, the drilling operation may sustain a loss of rate of penetration, and additional damage to other cutting elements, should the broken cutting element contact a second cutting element.

Additionally, the generation of heat at the cutter contact point, specifically at the exposed part of the PDC layer caused by friction between the PCD and the work material, causes thermal damage to the PCD in the form of cracks which lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear. The thermal operating range of conventional PDC cutters is typically 750° C. or less.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In one aspect, embodiments disclosed herein relate to a cutting element assembly that includes a rotatable cutting element having an axis of rotation extending axially therethrough, a cutting face and a body extending axially downward from the cutting face, at least one retention mechanism disposed adjacent to the rotatable cutting element, and a first magnet disposed at a back face of the rotatable cutting element.

In another aspect, embodiments disclosed herein relate to a downhole cutting tool that includes a body, a plurality of blades extending radially from the body, a plurality of cutter pockets disposed on the plurality of blades, at least one cutting element assembly disposed in the cutter pockets, wherein the at least one cutting element assembly includes a rotatable cutting element retained in the at least one cutter pocket, the rotatable cutting element comprising an axis of rotation extending axially therethrough, a cutting face and a body extending axially downward from the cutting face, and a first magnet disposed at a back face of the rotatable cutting element.

In yet another aspect, embodiments disclosed herein relate to a downhole tool that includes at least one dynamic component, at least one magnet disposed adjacent to the at least one dynamic component, and a ferrofluid adjacent the at least one magnet and a portion of the at least one dynamic component.

Other aspects and advantages of the disclosure will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a side view of a conventional drill bit.

FIG. 2 shows a bottom view of a conventional drill bit.

FIG. 3 shows a cross sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 4 shows a cross sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 5 is a picture of a partially disassembled cutting element assembly according to embodiments of the present disclosure.

FIG. 6 shows a cross sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 7 shows a test for retention of ferrofluid on cutting element assemblies according to embodiments of the present disclosure.

FIG. 8 shows a test for retention of grease on a rotatable cutting element.

FIG. 9 shows a test for retention of ferrofluid within a cutting element assembly according to embodiments of the present disclosure.

FIG. 10 shows a test for retention of ferrofluid within a cutting element assembly according to embodiments of the present disclosure.

FIG. 11 shows a partially disassembled cutting element assembly according to embodiments of the present disclosure.

FIG. 12 shows a side view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 13 shows a cross sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 14 shows a cross sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 15 shows a partially assembled cutting element assembly according to embodiments of the present disclosure.

FIG. 16 shows a partially assembled cutting element assembly according to embodiments of the present disclosure.

FIG. 17 shows a cross sectional view of a cutting element assembly.

FIGS. 18-22 shows pictures of cutting element assembly failures.

FIG. 23 is a partial cross-sectional view of a roller cone drill bit.

FIG. 24 is a side view of a roller cone drill bit.

FIG. 25 is a top view of a roller cone drill bit.

FIG. 26 is a cross-sectional view of a rotatable structure mounted to a drill bit according to embodiments of the present disclosure.

FIG. 27 is cross-sectional view of a rotatable structure mounted to a drill bit according to embodiments of the present disclosure.

FIG. 28 shows a front view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 29 shows a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 30 shows a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 31 shows a front view of a cutting element assembly according to embodiments of the present disclosure.

FIG. 32 shows a side view of a roller reamer according to embodiments of the present disclosure.

FIG. 33 shows a cross-sectional view of the roller reamer of FIG. 32.

FIGS. 34A-F illustrates the operation of a rotary steerable device within a borehole to steer a drill bit coupled to the rotary steerable device

FIG. 35 illustrates a rotary steerable device according to embodiments of the present disclosure.

DETAILED DESCRIPTION

Embodiments disclosed herein relate generally to rotatable structures including roller cones and rotatable cutting elements, also referred to as rolling cutters, and methods of retaining such rotatable structures on a drill bit or other downhole tools. For example, according to embodiments of the present disclosure, a magnetic force or magnetic field may be used alone or in combination with other retention mechanisms to retain a rotatable structure to a drill bit. Advantageously, retaining mechanisms described herein allow a rolling cutter to rotate as it contacts the formation to be drilled, while at the same time retaining the rolling cutter on the drill bit. A magnetic force or field may be used to provide additional features, such as providing a dampening effect to a rotatable structure, to help retain a ferrofluid within a bearing space of a rotatable structure, and to aid in rotation of a rotatable structure.

According to embodiments of the present disclosure, one or more magnets may be used to retain a rotatable cutting element to a drill bit or other cutting tool by placing the one or more magnets along the rotatable cutting element axis of rotation and opposite from the cutting face of the rotatable cutting element. For example, FIG. 3 shows a cross sectional view of a rotatable cutting element retained in a cutter pocket of a cutting tool according to embodiments of the present disclosure. As shown, the rotatable cutting element 300 has a cutting face 302 and a body 304 extending axially downward from the cutting face along an axis of rotation 306. The body 304 has a shaft 308 portion with a diameter smaller than the diameter of the cutting face 302. The shaft 308 of the rotatable cutting element 300 is disposed in a sleeve 310. The rotatable cutting element 300 and the sleeve 310 are disposed in a cutter pocket 320 formed in a cutting tool 325. A magnet 330 is disposed at a back face 303 of the rotatable cutting element 300 and adjacent to a bottom side 322 of a cutter pocket 320. The bottom side 322 of the cutter pocket 320 may be formed of a ferromagnetic material 324 such as steel. The magnetic force from the magnet 330 may pull the rotatable cutting element 300 into the sleeve 310 and cutter pocket 320, thereby tightening any gap 315 formed between the rotatable cutting element 300 and the top portion of the sleeve 310. Further, by pulling the rotatable cutting element 300 into the sleeve 310, the magnetic force may maintain full contact between the sleeve 310 and the rotatable cutting element 300, and thereby may prevent any foreign particles from getting into the cutting element assembly.

Further, the magnetic force may be used alone or in addition to other retention methods. For example, a rotatable cutting element may be retained in a sleeve using at least one retention mechanism disposed between the rotatable cutting element and the sleeve and at least one magnet disposed at the back face of the rotatable cutting element. Referring again to FIG. 3, a retaining ring 340 is disposed between the rotatable cutting element 300 and the sleeve 310 and may be used as a retention mechanism in addition to the magnet 330 to retain the rotatable cutting element 300. Particularly, the retaining ring 340 may be disposed in a circumferential groove 309 is formed in the outer surface of the shaft 308 of the rotatable cutting element 300. The circumferential groove 309 may have a height that extends axially along the shaft 308 and a depth that extends radially into the shaft 308. Further, the retaining ring 340 may protrude radially from the circumferential groove 309 into the sleeve 310.

According to embodiments of the present disclosure, a rotatable cutting element assembly may be retained directly to a cutter pocket formed in a cutting tool, without the use of a sleeve. For example, U.S. Provisional Application No. 61/834,264 and U.S. Publication No. 2011/0297454, which are incorporated herein by reference, include embodiments of retaining a cutting element directly to a cutter pocket.

FIGS. 28 and 29 show a front view and cross-sectional view of a rotatable cutting element assembly 280 according to embodiments of the present disclosure that is retained in a cutter pocket 285 using a retention element 286, or blocker. Particularly, the cutter pocket 285 may be formed along a side of a cutting tool 281, such that once the rotatable cutting element assembly 280 is disposed in the cutter pocket 285, the rotatable cutting element assembly 280 is partially exposed out of the cutter pocket 285. The rotatable cutting element assembly 280 includes a rotatable cutting element 282, a magnet 284, and the retention element 286. The magnet 284 is disposed between a bottom side 283 of the cutter pocket 285 and a back face of the rotatable cutting element 282 and along the axis of rotation of the rotatable cutting element 282. The bottom side of the cutter pocket 285 may be formed of steel or a ferromagnetic material 288. The retention element 286 may be placed adjacent to the rotatable cutting element 282, opposite the magnet 284, such that the retention element 286 covers a portion of the cutting face of the rotatable cutting element 282. The end of the retention element 286 that does not cover a portion of the rotatable cutting element 282 may be attached to the cutting tool 281, for example, by a screw, pin, other mechanical attachment means, or by brazing.

FIGS. 30 and 31 show a cross-sectional view and a front view of a rotatable cutting element assembly according to other embodiments of the present disclosure that is retained in a cutter pocket using a retention element. As shown, a blade 310 on a drill bit or other downhole cutting tool (including, for example, primary blades and secondary blades) provides a cutter-supporting structure to which rotatable cutting elements 360 are mounted. The rotatable cutting elements 360 are disposed/retained within cutter pocket 312, which may limit the lateral movement of the rotatable cutting element 360. Axial movement of the rotatable cutting element 360 out of the cutter pocket 312 is limited by retention element 362, which is disposed in a retention pocket 314 radially adjacent to and intersecting a portion of cutter pocket 312, and a magnet 320. In one or more embodiments, the retention pocket 314 extends around a partial circumference (ranging for example an arc length of less than 180 degrees, or from 60 to 120 degrees or 75 to 105 degrees in other embodiments) as well as a partial axial length of the cutter pocket 312. In one or more embodiments, both retention pocket 314 and cutter pocket 312 interface a leading face 316 (i.e., facing in the direction of rotation of the bit or other tool) of blade 310. However, it is envisioned that the retention pocket 314 may also be formed on a trailing face of the blade 310 and extend into blade 310 to interface cutter pocket 312 and rolling cutter 360.

The retention element 362 may be retained within retention pocket 314 by a screw 364 or other fastener, or may be brazed in place in yet other embodiments. In the illustrated embodiment, the screw 364 is inserted through a thru-hole in the retention element 362 and engages with blade 310 (such as by threaded engagement) or a threaded bolt infiltrated into blade 310. The rotatable cutting element 360, magnet 320, and retention element 362 may be assembled together in the cutter pocket 312 by disposing the magnet 320 at the bottom side of the cutter pocket 312, fitting a projection or lip 368 formed in the retention element 362 into a corresponding groove 366 formed around the rotatable cutting element 360, placing the rotatable cutting element 360 and retention element 362 into the cutter pocket 312 (and retention pocket 314) to cover the magnet 320, and finally, securing the retention element 362 to the cutting tool 310. By mating the lip 368 formed in the retention element 362 with the groove 366 formed around the rotatable cutting element 360, the retention element 362 may help to axially retain the rotatable cutting element 360 within the cutter pocket 312 without covering a portion of the cutting face 370 of the rotatable cutting element 360.

Various retention mechanisms may be used in combination with a magnetic force retention mechanism described herein. For example, a retention mechanism may include retention balls, springs, pins, retaining rings, or mating non-planar geometry. Retention mechanisms using retention balls may have a plurality of retention balls disposed between corresponding grooves formed around the outer surface of the rotatable cutting element body and the inner side surface of a sleeve, which is attached to a cutter pocket. Retention mechanisms using springs may include at least one spring and at least one ball or pin disposed between at least one blind hole and/or groove, such that the retention mechanism may be compressed when the rotatable cutting element is being fitted into the sleeve and may expand into the corresponding blind holes and/or grooves to retain the rotatable cutting element in a certain axial position within the sleeve. Retention mechanisms using mating non-planar geometry may include at least one corresponding groove and protrusion formed in an inner surface of the sleeve and an outer side surface of the rotatable cutting element. In such embodiments, the sleeve may be formed by joining two or more pieces together around the rotatable cutting element. For example, a rotatable cutting element may have a groove and/or a protrusion formed around its circumference. A sleeve having a mating protrusion and/or groove formed around the inner surface of the sleeve may be split along the length of the sleeve into at least two pieces. The at least two pieces may be assembled around the inner cutter such that the mating groove(s) and protrusion(s) are aligned, and the at least two pieces may be bonded together.

In other embodiments, a rotatable cutting element may be rotatably retained in a cutter pocket using changes in the rotatable cutting element body's diameter. For example, a rotatable cutting element body may have a first diameter proximate to the cutting end of the rotatable cutting element and a second diameter axially distant from the cutting end, wherein the second diameter is larger than the first diameter. A sleeve surrounding the rotatable cutting element body may have a first inner diameter corresponding with the first diameter of the rotatable cutting element. Thus, when the rotatable cutting element is assembled within the corresponding sleeve or pocket, the larger second diameter retains the rotatable cutting element. Various examples of retention mechanisms also include those disclosed in U.S. Patent Publication Nos. 2012/0132471, 2012/0273281, and 2010/0314176, and U.S. Pat. Nos. 7,703,559 and 8,091,655, all of which are assigned to the present assignee and herein incorporated by reference in their entirety.

Referring again to FIG. 3, the cutting face 302 of the rotatable cutting element 300 may be formed of diamond or other ultra-hard material. For example, a diamond material may extend a thickness of about 0.06 inches to about 0.15 inches from the cutting face into the rolling cutter, across the entire cutting face to form a diamond cutting table (not shown). In other embodiments, a rotatable cutting element may have a diamond or other ultra-hard material table having a thickness ranging from about 0.04 to 0.15 inches. Further, the cutting face may have a chamfer formed around the outer circumference, wherein the chamfer is not considered when measuring the thickness or diameter of the cutting table.

A diamond or other ultra-hard material table forming the cutting face of a rotatable cutting element may be disposed on a substrate formed of a variety of hard or ultra hard particles. In one embodiment, the substrate may be formed from a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the substrate, such as cobalt, nickel, iron, metal alloys, or mixtures thereof. In the substrate, metal carbide grains are supported within the metallic binder, such as cobalt. Additionally, the substrate may be formed of a sintered tungsten carbide composite structure. It is well known that various metal carbide compositions and binders may be used, in addition to tungsten carbide and cobalt. Thus, references to the use of tungsten carbide and cobalt are for illustrative purposes only, and no limitation on the type substrate or binder used is intended. In another embodiment, the substrate may also be formed from a diamond ultra-hard material such as polycrystalline diamond and thermally stable diamond. While the illustrated embodiments show the cutting face and substrate as two distinct pieces, one of skill in the art should appreciate that it is within the scope of the present disclosure the cutting face and substrate may be integral, identical compositions. For example, some embodiments may have a single diamond composite forming the cutting face and substrate or distinct layers. In some embodiments having diamond at the cutting face, the diamond may be layered or form a gradient having different diamond size and/or composition. For example, diamond at the cutting edge (around the circumference of the cutting face) may have a relatively finer grain size with a relatively higher wear resistance than the diamond away from the cutting edge having a coarser grain size with a higher toughness.

According to embodiments of the present disclosure, a magnet may be at least partially disposed within a sleeve or may be disposed axially behind the sleeve. For example, as shown in FIG. 3, the magnet 330 is positioned along the bottom side 322 of the cutter pocket 320 and within the sleeve 310. Referring now to FIG. 4, a cutting element assembly according to embodiments of the present disclosure includes a magnet 430 positioned between the bottom side 422 of a cutter pocket 420 and the back face 403 of a rotatable cutting element 400. The bottom side 422 of the cutter pocket 420 may include a ferromagnetic material 424. The rotatable cutting element 400 is disposed within a sleeve 410, and the magnet 430 is positioned axially behind the sleeve 410 along the axis of rotation 406. FIG. 5 shows another view of a magnet 530 positioned axially behind a sleeve 510 disposed in a cutter pocket 520.

According to embodiments of the present disclosure, one or more magnets may be used in combination with a ferrofluid in a bearing system for a rotatable structure. A ferrofluid may include ferromagnetic nanoparticles disposed in a fluid. For example, ferromagnetic nanoparticles may include coated nanoparticles of ferrite. The fluid carrying the ferromagnetic nanoparticles may include regular or high temperature grease used in downhole drilling operations. By using ferrofluid in combination with at least one magnet in rotatable structure assemblies of the present disclosure, the ferrofluid may accumulate around the at least one magnet and rotatable structure to provide damping for the rotatable structure and thereby reduce friction. For example, ferrofluid disposed around at least one magnet used in a cutting element assembly may provide damping between the rotatable cutting element and surrounding sleeve, and thereby reduce the amount of friction occurring during rotation of the rotatable cutting element within the sleeve. Ferrofluid disposed within a bearing space of a rotatable structure assembly of the present disclosure may also act as a coolant for frictional heat caused by the rotation of the rotatable structure. Further, the magnetic field from magnets used with rotatable structure assemblies of the present disclosure may keep ferrofluid from leaking out of the cutting element assemblies, especially when the grease/fluid loses viscosity at elevated temperature.

According to embodiments of the present disclosure, a ferrofluid may be disposed around at least one magnet in a cutting element assembly. For example, referring to FIG. 6, a rotatable cutting element 600 has a cutting face 602 and a body 604 extending axially downward from the cutting face along an axis of rotation 606. The body 604 has a shaft 608 portion with a diameter smaller than the diameter of the cutting face 602, which is disposed in a sleeve 610. The rotatable cutting element 600 and the sleeve 610 are disposed in a cutter pocket 620 formed in a cutting tool 625. A magnet 630 is positioned at a back face 603 of the rotatable cutting element 600 and adjacent to a bottom side 622 of a cutter pocket 620. The bottom side 622 of the cutter pocket 620 may be formed of a ferromagnetic material 624 such as steel. The magnetic force from the magnet 630 may pull the rotatable cutting element 600 into the sleeve 610 and cutter pocket 620, thereby tightening any gap 615 formed between the rotatable cutting element 600 and the top portion of the sleeve 610. A ferrofluid 650 is disposed around the magnet 630 and between the sleeve 610 and rotatable cutting element 600. Using ferrofluid in cutting element assemblies of the present disclosure may provide lubrication and a compressible cushion behind the rotatable cutting element. Further, a retaining ring 640 is disposed between the rotatable cutting element 600 and the sleeve 610 and may be used as a retention mechanism in addition to the magnet 630 to retain the rotatable cutting element 600. The retaining ring 640 may be disposed in a circumferential groove 609 formed in the outer surface of the shaft 608 of the rotatable cutting element 600 and may protrude radially from the circumferential groove 609 into the sleeve 610.

Referring now to FIGS. 7 and 8, rotatable cutting elements having ferrofluid and grease were submerged in water and placed in an ultrasonic cleaner for about an hour. Particularly, FIG. 7 shows a rotatable cutting element 700 having a magnet 710 disposed at the back face 702 of the rotatable cutting element 700. Ferrofluid 720 is disposed at the back face 702 of the rotatable cutting element 700 around the magnet 710. The rotatable cutting element 700, magnet 710 and ferrofluid 720 assembly were submerged in a container of water 730 and placed in an ultrasonic cleaner for about an hour. As shown, ferrofluid 720 remained around the magnet 710 after removing the rotatable cutting element 700 from the container of water 730. FIG. 8 shows a rotatable cutting element 800 without a magnet having a grease material 810 disposed at the back face 802 of the rotatable cutting element 800. The rotatable cutting element 800 and grease 810 were submerged in a container of water and placed in an ultrasonic cleaner for about an hour. As shown, no grease 810 remained around the rotatable cutting element 800 after removing the rotatable cutting element 800 from the container of water.

FIG. 9 shows a process of testing cutting element assemblies of the present disclosure. As shown, a cutting element assembly 900 includes a rotatable cutting element 910, a magnet 920 disposed at the back face of the rotatable cutting element 910 and adjacent to a steel material 950, a nonmagnetic sleeve 930, and ferrofluid 940 disposed around the magnet 920 and between the rotatable cutting element 910 and the sleeve 930. The cutting element assembly 900 is submerged in a container of water 960 and subjected to ultrasonic cleaner for about 45 minutes. Upon removing the cutting element assembly 900 from the container of water 960, ferrofluid 940 remains disposed around the magnet 920 and between the rotatable cutting element 910 and the sleeve 930.

FIG. 10 shows another process of testing cutting element assemblies of the present disclosure. As shown, a cutting element assembly 1000 includes a rotatable cutting element 1010, a magnet 1020 disposed at the back face of the rotatable cutting element 1010 and adjacent to a steel material 1050, a nonmagnetic sleeve 1030, and ferrofluid 1040 disposed around the magnet 1020 and between the rotatable cutting element 1010 and the sleeve 1030. The cutting element assembly 1000 is placed in a container 1060 of mineral oil and confined pressure. For example, the cutting element assembly 1000 may be subjected to a pressure of 500 psi for 15 minutes and 1,000 psi for 1 hour. Upon removing the cutting element assembly 1000 from the container 1060, ferrofluid 1040 remains disposed around the magnet 1020 and between the rotatable cutting element 1010 and the sleeve 1030.

As mentioned above, magnets used in cutting element assemblies according to embodiments of the present disclosure have a magnetic force that may be used to attract and hold ferrofluid within the cutting element assembly. For example, FIGS. 11-13 show various views of a cutting element assembly according to embodiments of the present disclosure having ferrofluid pulled around a magnet disposed at the back face of a rotatable cutting element. Particularly, FIG. 11 shows a partially disassembled cutting element assembly including a rotatable cutting element 110, a magnet 112 disposed at a back face 111 of the rotatable cutting element 110, and ferrofluid 114 disposed around the magnet 112. As shown, the ferrofluid 114 is held as a layer around the magnet 112. FIG. 12 shows a cutting element assembly including a rotatable cutting element 120, a magnet 122 disposed at a back face of the rotatable cutting element 120 and adjacent to a steel material 126, a nonmagnetic sleeve 128, and ferrofluid 124 disposed around the magnet 122 and between the rotatable cutting element 120 and the sleeve 128. The magnet 122 may attract the ferrofluid 124, thereby reducing leakage of the ferrofluid from any gap 125 formed between the upper portion of the sleeve 128 and the rotatable cutting element 120. FIG. 13 shows a cross sectional view of a cutting element assembly including a rotatable cutting element 130, a magnet 132 disposed at a back face 131 of the rotatable cutting element 130, a sleeve 138, a retention mechanism 139 disposed between the rotatable cutting element 130 and sleeve 138, and ferrofluid 134 disposed around the magnet 132 and between the rotatable cutting element 130 and the sleeve 138. As shown in FIGS. 11-13, the magnetic force created by the magnets may be used to attract and hold ferrofluid within the cutting element assembly around the magnets. The ferrofluid accumulated around a magnet may provide damping for the rotatable cutting element. The damping effect provided by the accumulated ferrofluid at the back face of the cutting element may help improve rotation of the rotatable cutting element.

Ferrofluid may be used in combination with at least one magnet with other rotatable structure assemblies on drill bits to provide at least one of the advantages described herein. For example, according to embodiments of the present disclosure, a drill bit may have a bit body and at least one rotatable structure retained to a journal extending from the drill bit. At least one bearing system may be disposed between the at least one rotatable structure and the journal to allow rotation of the rotatable structure around the axis of journal. The bearing space may include at least one magnet and a ferrofluid disposed between the at least one rotatable structure and the journal.

FIGS. 23-25 show an example of a drill bit having at least one rotatable structure, which may be referred to as a roller cone in this embodiment, retained to a journal extending from the bit. Particularly, FIG. 23 shows a cross-sectional view, FIG. 24 shows a perspective view, and FIG. 25 shows a top view of a drill bit 10 having a bit body 12 composed of three sections, or legs 17, that are joined together to form bit body 12. A roller cone 14 is rotatably mounted on a journal 18 extending downward and radially inward from each of the legs toward the center of the bit 10. Cutting elements 26 are disposed around the surface of the roller cone 14. A bearing system is disposed between the journal 18 and roller cone 14, and may include bearing surfaces and various bearing components disposed within the bearing space. For example, as shown, the roller cone 14 includes a bore 28 having a bearing surface 30 and an end surface 31, which mate with a journal bearing surface 42 and journal end surface 44 when the roller cone 14 is mounted to the journal 18. The bearing system may also include bearing sleeves 48, 49, disposed in grooves formed in the journal bearing surface 42 and the bore bearing surface 30, respectively, to help to reduce friction between the roller cone 14 and journal 18. A sealing assembly 50 may be disposed in a seal recess 34 formed in the bore bearing surface 30 to seal the bearing system. A nose bushing 45 may be disposed around a nose portion 47 of the journal 18, and a nose button 46 may be positioned between the journal end surface 44 and end surface 31 of the roller cone 14. The bearing system may be lubricated by grease, which may be supplied by a grease reservoir 19 including a pressure compensation subassembly 29 and a lubricant cavity 20, and connected to the ball passageway 36 by a lubricant passageway 21. The grease may be sealed in the bearing system and various connecting passageways 21, 35 by the sealing assembly 50. According to embodiments of the present disclosure, the bearing system may also include at least one magnet and a ferrofluid disposed between the journal 18 and the roller cone 14.

Further, the roller cone 14 is retained to the journal 14 using a retention ball system. As shown, the retention ball system includes corresponding circumferential grooves 32, 43 formed in the bore bearing surface 30 and the journal bearing surface 42, respectively, to form a race that receives and holds a plurality of retention balls 37. The retention balls 37 may be inserted into the race via a ball passageway 36 formed through the journal 18 to intersect with the race. A ball retainer 39 is disposed within the ball passageway 36, and a ball hole plug 38 is secured behind the ball retainer 39 to close the ball passageway 36 and keep the retention balls 37 within the race.

According to embodiments of the present disclosure, ferrofluid may be used in addition to grease in a bearing system of a drill bit. For example, FIG. 26 shows an exploded cross-sectional view of a roller cone 14 rotatably mounted to a journal 18 extending from a drill bit. The roller cone 14 is retained to the journal 18 using a plurality of retention balls 37, which are kept between the roller cone 14 and journal 18 with a ball retainer 39 and ball hole plug 38. Grease 60 is supplied to the bearing system formed between the roller cone 14 and journal 18 through various connecting passageways, and may be sealed within the bearing system by a sealing assembly 50. The sealing assembly 50 includes an o-ring seal 52 disposed between the journal 18 and the roller cone 14. A magnet 54 and ferrofluid 56 may be disposed proximate to the o-ring seal 52 to form a secondary seal. The magnet 54 may be a ring magnet extending around the circumference of the journal 18.

According to some embodiments of the present disclosure, grease used in a bearing system of a drill bit may include a ferrofluid grease. For example, FIG. 27 shows a cross-sectional view of a roller cone 14 rotatably mounted to a journal 18 extending from a drill bit. The roller cone 14 is retained to the journal 18 using a plurality of retention balls 37, which are kept between the roller cone 14 and journal 18 with a ball retainer 39. Grease 60 is supplied to the bearing system formed between the roller cone 14 and journal 18 through various connecting passageways, and may be sealed within the bearing system by a sealing assembly 50, wherein the grease 60 is a ferrofluid grease made of ferropowder mixed with a grease composition. The sealing assembly 50 includes an o-ring seal 52 disposed between the journal 18 and the roller cone 14. A ring magnet 54 may be disposed proximate to the o-ring seal 52 and extend around the circumference of the journal 18 to help prevent the ferrofluid grease from leaking. A magnet 58 may also be disposed between the roller cone 14 and the journal 18 at the nose end 47 of the journal 18 to provide additional axial damping on the roller cone 14. The magnets 54, 58 used in the embodiment shown may also help keep the ferrofluid grease 60 between the bearing space formed between the roller cone 14 and journal 18.

Although roller cones are shown in FIGS. 23-27 as the rotatable structures mounted to a drill bit, other rotatable structures may be rotatably mounted to a drill bit using at least one magnet in combination with a ferrofluid within a bearing system as described above. For example, in some embodiments, at least one magnet and a ferrofluid may be used in a bearing system between a roller disc rotatably mounted to a drill bit. Roller discs are similar to roller cones in that they may be rotatably mounted to a drill bit on a journal and have cutting element disposed about its outer surface, but have a disc-like shape as opposed to the conical shape of a roller cone. Roller cones, roller discs, and other rotatable structures may have various shapes and sizes depending on, for example, the type of formation being cut, the type of drill bit, and the orientation of the rotatable structures.

According to embodiments of the present disclosure, ferrofluid used with rotatable structures may include ferrofluid grease and/or ferrofluid lubricant. Ferrofluid grease may be formed by mixing dry ferropowder with grease, such as a lithium and calcium based grease, and optimized for drill bit and/or borehole environment performances.

In some embodiments, a second magnet may be used adjacent to and in combination with a first magnet in a cutting element assembly to create a damping effect. For example, FIG. 14 shows a cross sectional view of a cutting element assembly having a first magnet and a second magnet according to embodiments of the present disclosure. The cutting element assembly includes a rotatable cutting element 140, a first magnet 142 disposed at a back face 141 of the rotatable cutting element 140, a second magnet 143 disposed adjacent to the first magnet 142 and opposite the back face 141, a sleeve 148, and a retention mechanism 139 disposed between the rotatable cutting element 140 and sleeve 138. The first magnet 142 and the second magnet 143 may have north (N) and south (S) poles, wherein the same pole of each magnet 142, 143 is positioned adjacent to each other. For example, as shown in FIG. 14, the north pole of the first magnet 142 is positioned adjacent to the north pole of the second magnet 143. However, in other embodiments, the south poles of two adjacent magnets may be positioned to face each other. By positioning the same poles of the first and second magnets adjacent to each other, the repelling forces may create a damping effect at the back of the rotatable cutting element.

Further, as shown in FIG. 14, the first magnet 142 is disposed within the sleeve 148 and the second magnet 143 is not disposed within the sleeve 148. However, in some embodiments, both a first magnet and a second magnet may be disposed within a sleeve along with part of a rotatable cutting element. In other embodiments, both a first magnet and a second magnet may be disposed outside of a sleeve. For example, in some embodiments, a rotatable cutting element may be partially disposed within a sleeve, a first magnet may be disposed at a back face of the rotatable cutting element and axially behind the sleeve, and a second magnet may be dispose adjacent to the first magnet and opposite the back face of the rotatable cutting element, such that the second magnet is also axially behind the sleeve.

Magnets used in embodiments of the present disclosure may include permanent magnets, electromagnets, or superconducting magnets, and may include other mechanisms to create a magnetic field or magnetize otherwise non-magnetic components, for example, through application of a magnetic coating on an otherwise non-magnetic component. Permanent magnets may include, for example, neodymium magnets (neodymium iron boron based), samarium cobalt, alnico (aluminum, nickel and cobalt based magnets), ceramic and ferrite materials. Further, permanent magnets may be formed, for example, by injection molding, which may include mixing magnetic material with a matrix/binder material, such as resin, vinyl, etc. In embodiments using electromagnets, the strength of the electromagnet may be tuned based on the amount of load applied to the rotatable cutting element during operation.

Further, the size of magnets used in embodiments of the present disclosure may vary. For example, FIGS. 15 and 16 show embodiments of the present disclosure using different sized magnets. Referring to FIG. 15, a rotatable cutting element 150 includes a cutting end 152 and a shaft 154 extending axially from the cutting end 152. The shaft 154 has a diameter 155 that is smaller than the diameter of the cutting end 152. A magnet 156 is disposed at the back face 158 of the rotatable cutting element 150 and is concentric with the axis of rotation of the rotatable cutting element 150. The magnet 156 has a diameter 157 that is smaller than the diameter 155 of the shaft 154. Referring now to FIG. 16, a rotatable cutting element 160 includes a cutting end 162 and a shaft 164 extending axially from the cutting end 162. The shaft 164 has a diameter 165 that is smaller than the diameter of the cutting end 162. A magnet 166 is disposed at the back face 168 of the rotatable cutting element 160 and is concentric with the axis of rotation of the rotatable cutting element 160. The magnet has a diameter 167 that is substantially equal to the diameter 165 of the shaft 164. According to embodiments of the present disclosure, other shapes and sizes of magnets may be used at the back face of a rotatable cutting element along the axis of rotation.

In some embodiments, one or more permanent magnets may be attached or joined to a substrate of a rotatable cutting element with, for example, an adhesive, solder, or low temperature braze, wherein the solder or braze alloy melting point are lower than the permanent magnet Currie temperature. In other embodiments, a portion of a substrate of a rotatable cutting element may be magnetized, such that a magnet is formed as part of the rotatable cutting element. For example, the portion of a carbide substrate forming the back face of a rotatable cutting element may be magnetized such that a magnet is formed as part of the rotatable cutting element at the back face of the rotatable cutting element.

Further, according to embodiments of the present disclosure, a sleeve used in cutting element assemblies may be formed from a variety of materials. In one embodiment, a sleeve used in a cutting element assembly may be formed of a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the sleeve, such as cobalt, nickel, iron, metal alloys, or mixtures thereof, such that metal carbide grains may be supported within the metallic binder. In a particular embodiment, the sleeve is a cemented tungsten carbide with a cobalt content ranging from 6 to 13 percent. It is also within the scope of the present disclosure that the sleeve may also include more lubricious materials to reduce the coefficient of friction. The sleeve may be formed of such materials in its entirety or have a portions thereof (such as the inner surface) including such lubricious materials. For example, the sleeve may include diamond, diamond-like coatings, or other solid film lubricant. In other embodiments, the sleeve may be formed of steel or alloy composites, such as alloy steels, carbon steel, nickel-based alloys, cobalt-based alloys, and/or high speed cutting tool steels.

Embodiments of the present disclosure may provide cutting element assemblies with a higher resistance to chipping and wear between a rotatable cutting element and sleeve when compared with cutting element assemblies that do not have at least one magnet. Particularly, FIG. 17 shows a cutting element assembly 170 that does not have at least one magnet used therein. As shown, the cutting element assembly 170 includes a rotatable cutting element 172 partially disposed within a sleeve 174. A retention mechanism 176 is disposed between grooves formed along axially corresponding positions in the rotatable cutting element 172 and sleeve 174. Due to manufacturing tolerances, a gap 178 is formed between the axially upper, or top, portion of the sleeve 174 and the overlapping portion of the rotatable cutting element 172. During operation, rocking and other movement along the gap may cause chipping and uneven wear between the sleeve 174 and rotatable cutting element 172. Further, the gap 178 may allow grease to escape from between the sleeve 174 and rotatable cutting element 172 while contaminates may enter between the sleeve 174 and rotatable cutting element 172. The size of a gap may be less than 0.02 inches, for example, between 0.005 and 0.015 inches.

FIGS. 18-22 show images of various wear and chipping failures experienced by cutting element assemblies that do not use a magnet. Particularly, the cutting element assembly 180 shown in FIG. 18 has chipping along the gap 185 between the rotatable cutting element 182 and sleeve 184. The cutting element assembly 190 shown in FIG. 19 has uneven wear along the gap 195 on the sleeve 194 due to uneven loads by the rotatable cutting element 192. FIG. 20 shows a cross sectional view of a sleeve 200 that was used on a cutting element assembly that did not have a magnet. The inner surface of the sleeve 200 has wear from contaminates that entered through a gap in the cutting element assembly. The cutting element assembly 210 shown in FIG. 21 has a rotatable cutting element 212 disposed in a sleeve 214, wherein the rotatable cutting element 212 experienced breakage. Particularly, the rotatable cutting element 212 broke along the portion of the rotatable cutting element exposed outside the sleeve 214 and the portion of the rotatable cutting element disposed within the sleeve 214. The cutting element assembly 220 shown in FIG. 22 has experienced erosion along the gap 225 formed between the rotatable cutting element 222 and the sleeve 224.

According to embodiments of the present disclosure, at least one magnet may be used with a rotatable structure assembly. For example, at least one magnet may be used in addition to one or more other retention mechanisms to hold a rotatable cutting element within a sleeve. The magnetic force or field created by the at least one magnet may be used to pull the rotatable cutting element into the sleeve, and thus maintain full contact with the sleeve and prevent any foreign particles from getting into the cutting element assembly. By pulling the rotatable cutting element within the sleeve, the magnetic force or field may also reduce erosion along the interface between the rotatable cutting element and sleeve. The magnetic force or field may also help prolong the fatigue life of the cutting element by providing more front impact support for the rotatable cutting element and less yanking movement on the rotatable cutting element.

According to embodiments of the present disclosure, a magnetic force or field positioned at the back of a cutting element assembly may be used to provide damping effects on the back of the rotatable cutting element, thereby also reducing friction between the rotatable cutting element and the sleeve. The magnetic force may be used to create damping effects by placing the same poles of two magnets adjacent to each other and/or by using a ferrofluid. For example, in embodiments having ferrofluid disposed around at least on magnet positioned at the back face of a rotatable cutting element in a cutting element assembly, the ferrofluid may be pulled around the at least one magnet to create a compressible damping layer. Using a magnet in combination with ferrofluid in a rotatable structure assembly may also provide a means of minimizing leakage of the ferrofluid. According to embodiments of the present disclosure, ferrofluid may act as a lubricant for the rotatable structure, such as within a bearing space of a rotatable structure assembly, a damper for a rotatable structure, and/or a coolant for frictional heat created during rotation of the rotatable structure.

Another rotatable cutting structure in which the ferrofluid and magnet may be used is a roller reamer, illustrated in FIG. 32. FIG. 32 depicts a perspective view of roller reamer 1100. In the depicted embodiment, roller reamer 1100 includes a downhole tool body 1110 having uphole and downhole threaded ends (not shown) suitable for connecting with a drill string (or other downhole tool string). The tool body is generally cylindrical and includes a plurality of circumferentially spaced fixed blades 1115 that extend radially outward from a tool axis 1102. Fluid courses 1105 (also referred to as flutes) located between the fixed blades 115 allow for the flow of drilling fluid along the exterior surface of the tool 1100. Each of the blades 1115 includes a roller assembly 1200 deployed in a corresponding axial recess 1120 of the tool body 1110. While sealed bearing roller reamer 1100 is shown in FIG. 32 as having a single roller assembly 1200, it will be understood that the disclosure is in no way limited to such an embodiment and that the sealed bearing roller reamer commonly includes a plurality of roller assemblies 1200 (e.g., three) deployed at substantially equal angular intervals about the tool body 1110.

FIG. 33 depicts a cross sectional view through the roller assembly 1200 depicted on FIG. 32. In the depicted example, roller assembly 1200 includes a cutter shell or roller shell 1210 deployed about a bearing pin 1220. As described in more detail below, the cutter shell 1210 is disposed to rotate about a central axis of the roller assembly 1200 with respect to the bearing pin 1220 (i.e., the cutter shell 1210 is deployed substantially coaxially about the bearing pin 1220 and is arranged and designed to rotate with respect to the bearing pin 1220 about the common axis). The first and second axial end portions 1221 and 1222 of the bearing pin 1220 are deployed in and supported by corresponding first and second retention blocks 1240,1241. Thrust washers 1245 are deployed axially between the cutter shell 1210 and the retention blocks 1240, 1241 thereby enabling the cutter shell 1210 to rotate substantially freely with respect to the retention blocks 1240, 1241. First and second wedge blocks 1260, 1261 are deployed axially between the corresponding retention blocks 1240, 1241 and shoulder portions of the reamer body 1110 (these shoulder portions are also referred to below as end walls 1122). Threadable engagement of jack bolts 1262 to the reamer body 1110 urges the wedge blocks 1260, 1261 radially inward and between the retention blocks 1240, 1241 and the reamer body 1110 causing a wedging action that secures the roller assembly 1200 in the axial recess 1120. However, it is also intended that other retention mechanisms or arrangements may be used without departing from the scope of the pre sent disclosure.

In the depicted example shown in FIG. 33, bearing pin 1220 includes a central chamber 1225. A pressure compensation piston 1227 divides the central chamber 1225 into first and second, grease and spring chambers 1224 and 1226. Grease may be injected into the grease chamber 1224 via one or more ports in plug 1246 thereby urging pressure compensation piston 1227 against the bias of spring 1229 (and into the spring chamber 1226). The spring chamber 1226 is in fluid communication with the borehole annulus via hollow set screw 1237 such that the pressure compensating piston 1227 is urged towards the grease chamber 1224 via both spring bias and the hydrostatic pressure of the drilling fluid. The grease in the grease chamber 1224 is therefore maintained at a pressure greater than or equal to hydrostatic pressure. Radial ports 1223 in the bearing pin 1220 communicate grease from the grease chamber 1224 to an annular region between an inner surface of the cutter shell 1210 and an outer surface of the bearing pin 1220. As those of ordinary skill in the art will readily appreciate, the grease is intended to maintain lubricity between the cutter shell 1210 and the bearing pin 1220, thereby promoting substantially frictionless rotation of the cutter shell 1210 during drilling. Thus, in accordance with the present disclosure, the grease chamber 224 may include a ferrofluid (containing grease and a ferropowder), and a magnet (not shown) may be disposed adjacent to the annulus between the cutter shell 1210 and the bearing pin 1220, to maintain the ferrofluid within the annular region. Using a magnet in combination with ferrofluid in a rotatable structure assembly may also provide a means of minimizing leakage of the ferrofluid. Thus, according to embodiments of the present disclosure, the ferrofluid may act as a lubricant for the cutter shell as it rotates around the bearing pin, such as within a bearing space of between cutter shell and bearing pin, and in combination with a magnet adjacent the bearing space, the ferrofluid lubricant may be retained in the bearing space.

However, the use of the ferrofluids of the present disclosure is not limited to rotatable cutting structures, such as rolling cutters, roller cones, or roller reamers. Rather, it is envisioned that the ferrofluids of the present disclosure may be used in combination with a magnet on any dynamic component of a downhole tool. For example, such downhole tools may include a percussion bit that includes grease between sliding or percussive surfaces, an indexable bit with indexable (but not freely rotating cutting structures), open bearing (sealless) bits or other tools), jars, reamers (including rolling reamers, expandable reamers with a movable arm, etc), rotating control devices, mills, drilling motors, stabilizers, measurement while drilling tools, logging while drilling tools, steering tools, turbines, alternators, production pumps, under-reamers, hole-openers, turbine-alternators, whipstocks (including adjustable whipstocks), bent subs, and the like. In addition, it is also envisioned that the ferrofluids may also be used in combination with static downhole applications in which grease is conventionally used, and which may benefit from the grease having reduced washout or reduced contamination between two static surfaces. Such examples may include tools in which grease is used for assembly and/or disassembly, including, for examples, turbines, threaded connections, packers, etc. In such applications, the combination of a ferrofluid as grease with a magnet may beneficially allow for the grease (ferrofluid) to remain, to a greater extent than conventional greases, in the desired space upon use of the tool. Specifically, the magnet may attract the ferrofluid grease so that the ferrofluid grease has a reduced amount washout or displacement as compared to conventional greases.

Another example of a downhole tool that may use the ferrofluids of the present disclosure is a rotary steerable tool. FIGS. 34A-F depict the operation of the rotary steerable device 2200 within a borehole 2211 to steer a drill bit coupled to the rotary steerable device 2200 in a negative x direction. The device 2200 includes a cylinder 2202 having a slot 2206 therein. A cam 2208 is received within the slot 2206. The cam 2208 can rotate about a pin 2210, as depicted by the dashed lines. In FIG. 34A, cylinder 2202 is rotated in a clockwise direction, while cam 2208 rotates in a counterclockwise direction. In FIG. 34B, as the cylinder 2202 and the cam 2208 continue to rotate in their respective directions cam, 2208 is brought into contact with the borehole 2211. Although the cam 2208 may initially slide against the borehole 2211, at a certain point, the angle of can 2208 with respect to the borehole 2211 increases so that a “non-slip” condition is created and the cam “grips” the borehole 2211. In FIG. 34C, cam 2208 is rotated to a fully extended position while the cam still grips the borehole 2211. The rotational inertia of the steering device 2200 and the BHA causes the cam 2208 to rotate around its center of rotation (i.e. the point of contact with the borehole 2211), which pushes the rotary steerable device 2200 and a drill bit coupled to the rotary steerable device 2200 in a negative x direction. In FIGS. 34D-34F, the cylinder 2202 and the cam 2208 continue to rotate in their respective directions before returning to position depicted in FIG. 34A.

Referring to FIG. 35, another embodiment of the invention provides a rotary steerable device 600 including a wear ring 612 surrounding cams 608 a, 608 b, 608 c, 608 d. Wear ring 612 allows for continuous and/or increased contact with borehole. Wear ring 612 can minimize wear of cams 608 a, 608 b, 608 c, 608 d and can minimize the infiltration of drilling cuttings into slot 606. To further inhibit the infiltration of drilling cuttings, the volume defined by wear ring 612 can be packed with a grease, such as the ferrofluid of the present disclosure. A magnet may be utilized in a region adjacent the grease, to attract the ferrofluid grease to prevent washout or displacement of the grease and minimize infiltration of cuttings into the slots. Optionally, a gasket (e.g. a rubber gasket) can be attached to the exterior of cylinder 602 and wear ring 612 to prevent infiltration of drilling cuttings and/or maintain proper lubrication of cams 608 a, 608 b, 608 c, 608 d.

Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. 

What is claimed is:
 1. A cutting element assembly, comprising: a rotatable cutting element comprising an axis of rotation extending axially therethrough, a cutting face and a body extending axially downward from the cutting face; at least one retention mechanism disposed adjacent to the rotatable cutting element; and a first magnet disposed at a back face of the rotatable cutting element.
 2. The cutting element assembly of claim 1, further comprising a sleeve, wherein the rotatable cutting element is partially disposed within the sleeve.
 3. The cutting element assembly of claim 2, further comprising a ferrofluid disposed between the rotatable cutting element and the sleeve.
 4. The cutting element assembly of claim 1, further comprising a second magnet disposed adjacent to the first magnet and opposite the back face.
 5. The cutting element assembly of claim 4, wherein the first magnet and the second magnet each have poles, and wherein the same poles of the first and second magnets are positioned facing each other.
 6. The cutting element assembly of claim 1, wherein the first magnet comprises an electromagnet.
 7. The cutting element assembly of claim 1, wherein the first magnet comprises a permanent magnet.
 8. The cutting element assembly of claim 3, wherein the ferrofluid comprises a plurality of ferromagnetic nanoparticles.
 9. A downhole cutting tool, comprising: a body; a plurality of blades extending radially from the body; a plurality of cutter pockets disposed on the plurality of blades; at least one cutting element assembly disposed in the cutter pockets, wherein the at least one cutting element assembly comprises: a rotatable cutting element retained in the at least one cutter pocket, the rotatable cutting element comprising an axis of rotation extending axially therethrough, a cutting face and a body extending axially downward from the cutting face; and a first magnet disposed at a back face of the rotatable cutting element.
 10. The downhole cutting tool of claim 9, wherein a bottom side of the cutter pocket comprises a ferromagnetic material.
 11. The downhole cutting tool of claim 10, wherein the cutting element assembly further comprises a sleeve and at least one retention mechanism disposed between the rotatable cutting element and the sleeve.
 12. The downhole cutting tool of claim 11, further comprising a ferrofluid disposed between the rotatable cutting element and the sleeve.
 13. The downhole cutting tool of claim 9, further comprising a second magnet disposed adjacent to the first magnet and opposite the back face.
 14. The downhole cutting tool of claim 13, wherein the first magnet and the second magnet each have poles, and wherein the same poles of the first and second magnets are positioned facing each other.
 15. The downhole cutting tool of claim 9, wherein the first magnet comprises an electromagnet.
 16. The downhole cutting tool of claim 9, wherein the first magnet comprises a permanent magnet.
 17. A downhole tool, comprising: at least one dynamic component; at least one magnet disposed adjacent to the at least one dynamic component; and a ferrofluid adjacent the at least one magnet and a portion of the at least one dynamic component.
 18. The downhole tool of claim 17, wherein the downhole cutting tool is a drill bit, comprising: a bit body; a journal extending from the bit body, wherein the at least one rotatable structure is retained to the journal; wherein the bearing system is disposed between the at least one rotatable structure and the journal; wherein the at least one magnet is disposed between the at least one rotatable structure and the journal; and wherein the ferrofluid is disposed between the at least one rotatable structure and the journal.
 19. The downhole tool of claim 20, wherein the bearing system comprises an o-ring seal and the at least one magnet comprises a ring magnet disposed proximate to the o-ring seal.
 20. The downhole tool of claim 20, wherein the ferrofluid is a ferrofluid grease comprising ferropowder and grease.
 21. The downhole tool of claim 17, wherein the tool comprises a reamer, an indexable bit, a percussion bit, a hole opener, a turbine, a drilling motor, a stabilizer, a measurement while drilling tool, a logging while drilling tool, a steering tool, turbine, an alternator, a production pump, an under-reamer, a hole-opener, a turbine-alternator, an adjustable whipstock, or a bent sub. 